Method for determining the subterranean extension of geologic bodies



U R aimimn [mum ay 4, 1948. F. w. LEE 2,440,693

METHOD FOR DETERMINING THE SUBTERRANEAN EXTENSION OF GEOLOGIC BODIESFiled May 29, 1942 3 Sheets-Sheet 1 EEDER/CK W LEE INVENTOR ATTORNEY@JKU KLJUM ay 4, 1948. w, LEE 2,440,693

METHOD FOR DETERMINING THE SUBTERRANEAN EXTENSION OF GEOLOGIC BODIESFiled May 29, 1942 3 Sheets-Sheet 3 HPEDER/CK W LEE INVENTOR ATTORNEYPatented May 4, 1948 METHOD FOR DETERMINING THE SUB- TERRANEAN EXTENSIONOF GEOLOGIC BODIES Frederick W. Lee, Owings Mills, Md.

Application May 29, 1942, Serial No. 445,000

(Granted under the act of March 3, 1883, as amended April 30, 1928; 3700. G. 757) 2 Claims.

The invention described herein may be made and used by and for theGovernment of the United States for governmental purposes without thepayment to me of any royalty therefor.

This invention relates to methods which are especially adapted todelineate geologic bodies using drill holes, mine shafts, drifts andadits in conjunction with the ground surface by electrical methods. Thiselectrical method is a further extension of the Lee partitioning method,U. S. Patent No. 1,951,760, granted March 20, 1934, and its widerapplication in U. S. Patent application Serial No. 333,752, filed May 7,1940, now Patent No. 2,345,608, granted April 4, 1944.

An object of this invention is to provide a new method and newcombinations of methods of measuring under certain specified conditions,with current and potential electrodes applied to the earth, groundcurrent and potential fields in a new manner enabling determination ofthe proximity and extension of subterranean geologic bodies in the fieldunder examination. By the use of new methods according to thisinvention, it is possible to determine the proper direction foroff-setting an oil, gas, water, salt, sulfur and like well, or theproper direction for drilling a second well in a better location, usingan exploratory well or Wells, previously drilled. The invention appliesin the same Way to mineral mining, for determining the extension of orebodies, using diamond drill holes, stopes, or other undergroundlocations such as old shafts, adits, drifts, and like means of access tosubterranean points, as centers for observation at which one or moreelectrical subsurface ground contacts can be obtained.

Other and further objects and advantages of the invention will beapparent from the following description of preferred modes of applyingit to terrain typically illustrative of that encountered in any form ofgeologic investigation, herein exemplified by oiland mineralbearinggeologic bodies. It will be understood that if one is searching for abody more or less conductive than the surrounding geologic formation,lower or higher resistivity indications, respectively, will be sought,and that when one of these characters of indication is referred tohereinafter, it is intended to exemplify the other as well.

In the accompanying drawings illustrative of certain preferredapplications of the invention:

Fig. 1 illustrates a general application of the invention to a geologicstructure having a ground surface S and providing access to thesubsurface, exemplified by well W;

Fig. 2 is a diagrammatic earth section taken along line l--2 of Fig. 1,showing isotropic subsurface conditions;

Fig. 3 is an orientation chart, showing a mode of shifting the line |-2of Fig. 1 into various azimuthal directions through the point of origin;

Fig. 4 is a vector or contour diagram of measurements made according toFigs. 2 and 3 under isotropic conditions;

Fig. 5 is a diagram similar to Fig. 2 showing anisotropic subsurfaceconditions;

Fig. 6 is a plan view of Fig. 5;

Fig. '7 corresponds to Fig. 4 and shows the Warping eifect on the vectoror contour diagram of anisotropic conditions exemplified in Figs. 5 and6;

Figs. 8 and 9 are diagrams similar to Fig. 2, showing the Warping effecton the equipotential plane I9 of anisotropic conditions;

Fig. 10 is a diagram similar to Fig. 2, exemplifying the employment of aplurality of subsurface points of origin;

Fig. 11 shows a vector or contour diagram corresponding to Fig. 10;

Figs. 12 and 13 are diagrams similar to Fig. 2, of a modifiedapplication of the invention to isotropic and anisotropic terrain;

Fig. 14 is a vector or contour diagram corresponding to Fig. 13;

Figs. 15 and 16 are diagrams corresponding to Figs. 1 and 6,exemplifying examination of vertically oriented geologic bodies by meansof transverse impedisivity measurements obtainable by the method of thisinvention;

Fig. 17 is a vector or contour diagram of transverse impedisivitymeasurements corresponding to Figs. 15 and 16; and

Fig. 18 is a diagram corresponding to Fig. 2 showing a preferred mode ofpracticing the invention with markedly sloping terrain.

In my prior Patent No. 1,951,760, a center point of origin (Po herein),shown at substantially the same elevation as a straight base lineconnecting the current electrodes, C102, is employed as a reference, andthe potentials relative thereto of further potential points, PiPz,likewise shown at substantially the same elevation as the straight baseline, are measured. When such measured values, read with differentazimuthal directions of the base line and using a constant value ofground current are plotted, using a. sufficient number of azimuthaldirections to approximate a continuous locus for the value of the vector(two values of which, apart, are read as POPI and PoPz with each baseline orientation), then if the geologic body within the ground currentfield were isotropic, a circular locus, as b in Fig. 4 herein, would bevectorially described. This circular locus in effect, indicates that thesame number of equipotential shells (regarded as separated by definiteconstant differences of potential per unit of current passing in theearth) have been created in the earth between the points P and thedifferently azimuthally located points P1 (i. e. P2). These shells maybe visualized under ideal isotropic conditions as harmonic surfacesapproximating hemispheres and lying within the limiting cases of (1) aplane surface half way between the two current electrodes (i. e.hemispheres with infinite radius) and (2) hemispheres of extremely smallradius concentric with the current electrodes themselves. It will ofcourse be understood that when the term equipotential plane is employedherein, it connotes near-planar equipotential shells, because whenanisotropic conditions are present, either in the earth, by virtue ofdifferences in ground-contact resistance effects, or by virtue of lackof symmetry of the configuration, the ideal electrical distribution canonly be closely approximated.

The presence of anisotropic conditions within the current field affectsthe current distribution in the earth, modifies the shape of theequipotential harmonic surfaces, and displaces them so that the samenumber of shells will not necessarily appear between Po and P1 indifierent azimuthal positions of the latter.

Thus, if the vector values POPl (i. e. PoPz) are measured in terms ofpotentials per unit of current flowing in the earth, to indicate thenumber of shells which have been forced into the regions embraced by P0and P1 in various azimuthal positions of the latter, different values ofthe shell-indicating potential differences POPl will be found in thedifferent azimuthal direction when anisotropic conditions are present. Atypical resulting anisotropic vector diagram for the potentialdifferences PoP1 under constant current conditions at substantially thesame elevation as the base line, is shown in Fig. 7, vector locus 330..

Further, as explained in my above-mentioned application, when theterrain is isotropic horizontally, that is, in laminae parallel to thesurface, if further electrodes P3 and P4 are located on a line normal tothe center point P0 of the base line, they will lie on the sameequipotential plane as the center point P0 and no transverse componentof potential will appear between points on that equipotentlal plane.However, if anisotropic conditions exist, as in the case of steeplydipping stratified beds or intrusive geologic bodies, such as dikes orveins, faults and the like, the equipotential shells are warped and thepoints P0, P3 and P4 will no longer lie on the same equipotential planeor shell, except in very unusual cases, and the potentials appearingbetween P0 and P3 and between Po and P4, reflecting the number of shellsfalling between them, give data indicating the presence and character ofthe anisotropic material in the region investigated.

In all of the above instances, when the potentials are measured betweenpoints at substantially the same elevation as the current base-line, thedata obtained is that for an average of conditions existing between thebase-line and the full depth of any substantial current conduction bythe earth.- This depth is generally considered to be of the order Ofone-third the distance between the two current electrodes C1 and C2 (seeExploration Geophysics, J. J. Jakosky, 1st ed. (1940) Times-MirrorPress, Los Angeles, Calif., page 325). As a result it has heretoforebeen necessary, to obtain indications of the depth of anomalies, toconduct repeated re-investigations of the same regions with differentelectrode spacings in order to determine at what depth, as indicated byelectrode spacings, the effect of an anomaly was first encountered as achange in the average indicated conditions from the surface to suchdepth.

Then it was possible, by repeated test at various stations, to determinewhether the anomaly continued to make its presence more or less evidentin the average figures reflected by the surface potential loci diagrams,and by repeatedly varying th size Of the configuration, and itsazimuthal orientation, a general idea of the depth and direction ofsubsurface anomalies could be obtained, as explained in my aboveidentified application.

I have now discovered that it is possible to secure .such informationmore accurately and reliably, with the expenditure of much less time andeffort, by largely eliminating from the indications anomalies lyingclose to the surface Po point, and making measurements in a new mannerin which the effect of anomalies in a particular body lying at aconsiderable sub-surface depth is markedly increased as compared to theaverage of all overlying and underlying anomalies,

This better information can also be confirmed, and supplemented byinformation as to degree of extent of the anomaly, by combining with thedata obtained in the new subterranean steps, data resulting from certainmeasurements of surface potentials in accordance with my aboveidentified patent and application. The particular surface measurementsof greatest utility for supplementing the data obtained by the newsubterranean investigation step, to yield information not obtainable byeither the new subterranean or old surface measuring steps alone, areobtained by surface measurements with the same C1Cz, P1 and P2electrical locations employed in obtaining the particularized datayielded by the new subterranean mode of procedure.

My present invention thus rests on the discovery that by making thepotential point of origin P0 in the earth at an elevation removed fromthe elevation of the current contacts and instead located in theneighborhood of the particular strata or other geologic body underinvestigation, the effect on the measured values of the particulargeologic body may be enhanced, particularly if the remainder of the Leepartitioning method or five electrode method configuration (see Jakosky,cited above, page 326) is proportioned generally in a predeterminedmanner in accordance with my invention.

Similar, though less sensitive results may be achieved by locating theremainder of the configuration in other than the preferred symmetricalrelation to the point of origin Po, but as the symmetrical arrangementgives the best results, this arrangement is most completely describedherein as exemplifying the invention by the best mode known ofpracticing the same.

This preferred mode of practicing the invention is exemplified in Figs.1 to 11 and 15 to 18 herein. Referring to Fig. 1, W represents a well,

shaft, bore-hole or any other means of access to a geologic body,indicated at 3. In accordance with this embodiment. of the invention, apotential contact Po, intended to be the center of exploration, is madein the neighborhood of the region 3. A base-line point P0 preferablydesigned to be the center point of the base-line, is then selected lyingat a different elevation, which may be the surface of the earth, andpreferably located on the normal line extending from the point P0 to thecurrent base-line l-2 of the Lee or similar configuration.

For best sensitivity, current is then introduced into the earth throughcurrent electrodes C102 positioned on the current base-line l-2 onopposite sides of and at distances from the base-line center Po about 1times the distance PoPo', and reference potential contacts PlPZ are madein any desired manner, but preferably at the one-third points of theline C102 as shown.

Any desired source of current, either A.-C., D.C., or both, may besupplied to current electrodes C1C2; as from D.C. source 4, limitingresistor lllzb, current measuring device I, and re versing switch 9,through connecting switch 8 and impedance cont-roller H]; or as fromalternator 5, current measuring device 6 and resistance Ina throughconnecting switch 8 and impedance controller Ill. The illustratedarrangement of limiting resistor or impedance controllers i0, la and "lbis preferred, to enable separate adjustment of the A.-C. and D.C. valuesrelative to one another, and concurrent adjustment of both together.

Any desired means for measuring quantities reflecting the resistivity,or the earth potentials, at points PiPz, or when desired P3 and P4, orboth, relative to P0, and when desired relative to P0 for supplementaland confirmatory information, may be employed. In addition, the presenceof anisotropic conditions, as reflected by the warping of theequipotential plane or shell passing through P0, may be qualitativelychecked by the measurement of the potential difference existing betweenP0 and P0, or between Po and any other desired points located on thenormal line mentioned above.

To facilitate the making of the desired measurements, I prefer to employan arrangement as shown, in which the potential measuring instrument I4,with reversing switch I3, is connectable in any of the above-mentionedmodes, by means of selective switches II and I2.

If desired, the method and apparatus of my prior patent, No. 2,277,707,granted March 31, 1942, may be employed to advantage in practicing thepresent invention, in which case resistivity values may be measureddirectly, as the quantity above-mentioned.

Furthermore with the special switch 8 and control impedances Illa andlflb it is possible to apply both A.-C. and D.C. to the ground at thesame time. If D.C. is to be blocked from the A.-C. generator then (Bashould be a capacity If A.-C. is to be blocked from the D.C. generatoror battery then lOb should be an inductance choke. The effect ofapplying D.C. tends to build up dif ferences of potential betweendifferent geologic beds which, when measured with either D.C. or A.-C.facilitates differentiation as to the character of the bed contents.Conversely, the application of A.-C. under certain circumstances may beemployed to diminish the natural potentials between the beds tofacilitate differentiation in subsequent measurements with D.C. or A.-C.Therefore, for completeness, both A. C. and D. C. sources have beendisclosed in Fig. 1, but it should 'tials.

QlU-llibll RUUllll be appreciated that the depths at which observations'can ordinarily be made employing A. C., of even very low frequency, arehighly limited. Even low frequencies of alternating current produce thewell-known skin-effect which causes the current to concentrate near thesurface of the conductor, herein the earths surface. The dense flow ofcurrent at such=surface causes magnetic fields, which, as is well known,block flow of current in the body of the conductor remote from saidsurface. Therefore, while A. C. may be employed for observations closeto the surface, it does not produce equipotential shells normal to thecurrent paths at distances remote from the surface, as are produced whendirect current is used. Thus, when observations are to be made at anyconsiderable depth from the accessible region at which the currentelectrodes are placed, as a practical matter direct current must beemployed.

The current control If], shown as a resistance, serves to modify thecurrent as desired. It may be an impedance of any type known to the artwhich can control either A.-C. or D.C. or both. A potential measuringdevice, as a voltmeter, vacuum tube voltmeter, or potentiometer is shownat M. The reversing switch I3 in the potential circuit may be used incombination with reversing switch 9 in the current circuit. Thepotentiometer 14 may be selective, i. e., may respond to A.-C. only,using inductive coupling, or to D.C. only, by inductive chokes orDArsonval registration with long period movement; it may also beoscillographic, registering instantaneous poten- The same, of course,applies for current measuring means 6 and I.

With the arrangement shown in Fig. 1, applied to a region in which allstrata l6, ll, I8 are horizontally isotropic and indicated in Fig. 2,the uniformity of spacing of the current and potential contacts CICZPIPZfrom the center of exploration Po and from the comparison center Po,will result in equal potential drops, 1. e., resistivities, beingmeasured between Po on the one hand and P1 and P2 respectively, on theother; and the com-.

parison observations will likewise produce equal values, due to thesymmetry of the equipotential shells l4, l5 and 19 (Fig. 2).

If now the quantities appearing between Po on the one hand, and P1 andP2 on the other, be measured (and the check measurements be made betweenPo and P1 and P2 respectively) with the base-line CiC2 oriented inseveral different azimuthal directions 20, 2|, 22, as shown in Fig. 3,the values so determined may be plotted as vector lengths extending inthe respective azimuthal directions about the center of observation Po.As shown in Fig. 4, the locus of the ends of'these vector lengths(associated in accordance with this invention to constitute a contour ofthe direction and extent of the resistivity values at the elevation Pu),under isotropic conditions, will define a circle a (shown in solidlines) with its center at O', and the locus of the ends of the checkmeasurement vector lengths representing the values P0P1, P0P2 in theseveral directions 20, 2 I, 22, when associated in accordance with thisinvention, will define a-similar resistivity contour, again in the caseof isotropic material having the shape of a circle 17 (shown in dottedlines) about the point 0 as a center. In Fig. 4, for ease of comparison,the centers 0 and o are superimposed.

If, now, anisotropic conditions are present at or near the elevation ofthe center of observation Po, as shown in Figs. 5 and 6, in which thestrata l6 and I l are horizontally isotropic, while the stratum I8 isdivided at 25 into portions 26 and 21 having different electricalproperties, and readings are taken in various azimuthal orientations 28,29, 30 and 3| (Fig. 6) the vector diagram of quantities measured aboutPo (contour 33, Fig. 7) will no longer have a circular form, and thedirection and extent of anisotropy will be reflected in the elongationor foreshortening of the vector quantities in the directions from P inwhich such anomalies occur. For comparison the circular contour of adiameter k-n is superimposed in Fig. 7, on the contour of quantities 33measured about point P0, and on the contour of comparison values 33ameasured about Po, the contours being respectively vectorally plottedabout centers 0 and o, superimposed for ease of comparison.

As above-mentioned, the preferred position of the center of thebase-line 1-2, at which the comparison point P0 may be located, is onthe normal line from P0 to the base-line l2. The comparison may be madewhen necessary, with any point lying on the equipotential plane or shellpassing through point P0, and thus, when surface conditions arerelatively isotropic, P0 may be offset along the line |-2 (Fig. 1)extending perpendicularly to the base-line l2 and marking theintersection of the equipotential plane with the earth surface, or maybe positioned at any other accessible point in the equipotentialplane-defined by the line l'2' and the normal line extending therefromto the center of exploration Po.

Fig. 8 illustrates the effect on the potentials in the vicinity of P0and P0, of anisotropic conditions. Referring to the ideal isotropiccase, Fig. 2,

theoretically the mid-plane of the configuration will coincide with anddefine the equipotential plane of half -potential, and all points on theplane I 9 including Po and Po, would therefore, for ideal isotropicconditions have the same potential. When anisotropic conditions arepresent, as in Fig. 8, the reference plane I9. preferably at the centerof the configuration, will no longer be an equipotential plane. Thepresence of anomalies displaces the equipotential shells or loci, two ofwhich are represented at 34 and 35, so that the locus or plane of pointsequipotential with P0 will no longer, at the elevation of P0 cut throughthat point, but under the conditions assumed in Figs. -8, will cut thesurface at o'. The potential difference or other quantity measuredbetween Po and P1 (or P2) will no longer equal the correspondingquantity measured between Po and P1 (or P2), and to obtain equal valueswith surface measurements it would be necessary to move the point P0 to0, an indeterminate location in the absence of exploration about thepoint P0 in accordance with the present invention.

If the above-mentioned qualitative check is made by initially measuringthe potential difference or other quantity appearing between Po and Po,it will be observed that the fact of anisotropism will be quicklyestablished, and that by making the one measurement alone, with theconfiguration oriented in several azimuthal directions a quick estimatemay be made of the direction in which the displacement, P00 is greatestas a quick indication of the probable direction of extent of thegeologic body at or in the vicinity of P0.

In other words, merely measuring the potential difference (or relatedquantity, such as resistivity, for example) between Po and Po, for

different azimuthal orientation of the current configuration C1-C2 aboutthe axis PoPo', will, in the case of anisotropic material. show avariation of the potential difference with azimuth of current flowwhich, reduced to a contour as before mentioned, indicates by its majorand minor radii the directions of subterranean extension of bodies nearthe point P0.

As above-mentioned, the measurements from point P0 to points P1 and P2reflect not only anomalies lying at the point P0 (Fig. 8) but also thosein the vicinity thereof, as illustrated in Fig. 9, in which the effecton the equipotential shells exemplified at 34 and 35, of a nearby body36, is represented.

While greatest sensitivity is obtained with the current electrodes eachspaced from P0 about 1 /2 times the distance PoPo' and on opposite sidesthereof, valuable data may be collected at different depths of thecenter of observations, without moving the current electrodes C1C2 orthe potential electrodes PlP2. As shown in Fig. 10, the inventioncontemplates the employment of a plurality of subsurface points ofobservation, or an ambulant subsurface observation, represented at P0and P0". The qualitative indications may be quickly obtained bymeasuring potentials or quantities dependent thereon between P0 andobservation centers at various levels P0, P0" etc., if it is desired tofind a bed level at which the making of more extended observations maybe profitable. Furthermore, if, for example, one is interested in a bedat say the level Po, and cannot obtain a very great difference in vectorquantities between it and the corresponding vector quantities about thesurface comparison point P0, one may locate a subsurface point, say atP0 where the equipotential plane is locally considerably warped, and bymeasuring the potentials P0'-Po" in various azimuthal directions of thecurrent field, obtain a wider variation of potential (reflecting thewider variation between conditions near Po and P0", than that betweenthe condition at P0 and P0) to facilitate observations.

Preferably, however, arrangements as exemplified in Fig. 10 are employedwhen several oilbearing sands or other geologic bodies are beinginvestigated. Under those circumstances, as shown in Fig. 11, the vectordiagrams about check point P0, and observation points Po and P0", mayhave the forms shown at 39, 40 and 4|, respectively. When such diagramsare obtained, it will be clear that diagram 39 combines the effects ofthe anomalies at all levels within the general depth of observationestablished by the size of the configuration used, while diagram 40reflects predominantly the anomalies in the vicinity of P0, and diagram4! reflects predominately the anomalies in the vicinity of P0". Underthe conditions represented in Fig. 11, the higher of the two sands isseen to extend predominately in a northeasterly direction, the lower oneto extend in a southwesterly direction. Thus one would locate new wellsaccordingly in planning to tapthe desired sands. Diagram 39, whichindicates extension in both the northeast and southwest direction doesnot differentiate each sand separately, and does not indicate in whichdirection new wells should be located to tap a particular one of thesands.

While the use of symmetrical configurations is desirable from thestandpoint of ease and sensitivity, yielding two comparable azimuthalreadings in opposite directions for each electrode setminimum values.

up, the same principles may be employed with a non-symmetricalarrangement.

Fig. 12 illustrates such application in the case of ideal isotropicconditions. It will be observed that under such conditions, as theconfiguration is rotated in azimuth the vector quantities P1'P2 andP1-Pz will remain constant, respectively, and exhibit a constantpotential difference represented by the differences in potential betweenequipotential shells 8| and IS in the first case, and between shells l4and I5 in the second, while the qualitative indication Pi-Pi will have aconstant value for all azimuthal directions equal to the differencebetween the potential shells l4 and 8|.

With this application, under anisotropic conditions exemplified in Fig.13, the anisotropic body 36 causes warping of the equipotential shells,exemplified at a and b, and as the body 36 has different effects indifferent orientations of the configuration base-line C1Pz-C about theobservations points P1-P1', the quantity measured between Pi and P2 willhave different vector values in the different directions, as plottedabout P1, shown in Fig. 14, and interpretable as before. As in theprevious vector diagrams, check readings of the less sensitiveindications about P1 (not shown) may also be plotted, if desired.

The present mode of obtaining azimuthal extension indications especiallydependent on conditions at a desired sub-surface level is alsoapplicable to the transverse modes of impedisivity factor measuringdescribed in my copending application Serial No. 333,752, filed May 7,1940, as exemplified in Figs. 15 to 1'7 herein. As shown in Figs. 15 and16, suitable conditions for this application of my invention may beexemplified by a dike I8 of material to be investigated, strikingupwardly through laterally disposed materials 42 and 43, the wholecovered by an overburden 16.

To obtain indications of direction of strike and dip of such dike l8,much more sensitively and accurately than is possible while employingonly a surface electrode arrangement of the inventi-on of myabove-mentioned application, a new sub-surface center of observation Pomay be employed according to the present invention, and transverseimpedisivity reflecting quantities measured from the sub-surfaceelectrode P to potential electrodes P3 and P4 disposed on a line l'-2extending transversely to the current baseline C1C2.

Assume now, as shown in Fig. 16, that body [8 is divided by surface 2into two bodies 26 and 21, of which 26 is more resistant than 21, aswould occur if body 18 were a fault zone between geologic contacts 42and 43, and mineralized in 21 by a conducting ore and not so mineralizedin 26, and that bore hole W has struck into the non-mineralized region26.

Under such conditions the measurement of quantities appearing betweenP0, in the body l8, and electrodes P3 and P4 (transversely positionedrelative to the current base-line C1C2), in various azimuthal positionsof orientation of P3 and P4, (28, 29, 30, 3|. Fig. 16) and the chartingof the measured resistivities, resistances or potentials as vectorquantities in the corresponding directions 28, 29, 30, 3|, as in Fig.17, yields a locus or contour passing through the vector points P3P4having well defined maximum and Under these conditions the larger valuesoccur when the line of the potential ground contacts P3P4 is at 45 tothe extension of the geologic body and the minimum values MUUIW.

occur when the line of the potential contacts parallels or is at rightangles to the direction of extension of the body l8. Also the vectorvalues reflecting resistivity are higher in the direction of thenon-mineralized portion 26 of body l8, and lower in the direction of themineralized portion 21. That is, Po'P3 is greater than Po'--P4 onazimuths 29 and 3!, while only small residual values appear on azimuths28 and 30. The contour diagram, or the data supporting it, thusindicates that the high resistance body in question, 26, lies betweenthe larger projections of the locus for high resistivities, i. e.,between N. E. and N. W., and that the low resistance body inquestion,21, lies between the S. W. and S. E. directions, hereindirectly south. Thus by taking high values in sequence on the peripheryof the contour, or low values in sequence thereon, one is informed thatthe geologic body is defined between them.

As will be apparent from the above detailed description, if the earthssurface slopes as indicated in Fig. 18, the Center of the base line l-2will preferably be located at the point of intersection with the earthssurface of a line drawn from the subterranean contact P0 perpendicularto the plane of the earths surface (which perpendicular line may betermed the projection of the subsurface contact to the line connectingthe current contacts), and the surface configuration of contacts willpreferably be laid out or made symmetrically with respect to the pointof intersection of said projection with the earths surface.

In other words, regardless of the slope of the ground, I prefer to havethe current contacts define the base of an isosceles triangle having thesubterranean contact at its apex, and to make the potential measurements(which may be taken as representative of related quantities such asresistivity and the like) between the subterranean point on the onehand, and further potential contact points located symmetrically withrespect to the base of said isosceles triangle (as nearly as is allowedby the conditions of the terrain), substantially removed from theinfluence of the local conditions at the current contacts, andpreferably located on the base of the isosceles triangle at its centeror third points, or On a transverse plane perpendicular to the center ofsaid base, on the other hand.

Thus, in the preferred symmetrical form, the current electrodes C1-C2define the base of an isosceles triangle with P0 at its apex, which baseis preferably three times the height of the triangle along its apexmedian P0Po, and the potential contacts are located either on the baseof the triangle (as P0, P1 or P2) or on the plane determined by thepoint P0 and a line perpendicular to the mid-point of the base, as P0,P3, P4 or Po".

It will be appreciated from Fig. 11 that by combining with these newreadings, readings made about Po as a center in accordance with my priorpatent and application, it is possible to determine whether the latterreadings depart so markedly from similarity with the Po readings as toindicate that they reflect the presence of another strata or geologicbody. at a different elevation than Po, the presence of which may notpreviously have been known.

It will also be appreciated from Fig. 10, that by the making ofobservations at points represented by P0 and Po" traversing a givenstructure, the thickness of the structure may be closely determinedwithout the necessity of repeating observations with different currentelectrode spacings as heretofore practiced;

As will be evident from the foregoing disclosures, the present new modeof rendering geologic extension observations especially dependent onconditions at the particular subsurface depth being investigated may'beapplied to the several fields of application ofthe normal conductivitymeasuring configurations, and can be employed to obtain data as toconditions underlying highly conductive overburden such as salt waterbeds near the surface and the like,. which render the older surfaceobservation of average conditions valueless or highly unreliable becauseof the obscuring effect of the overlying beds.

The invention is thus not limited to the particular embodiments andapplications shown to illustrate its principles and practice.

I claim as my invention:

1. A method of geophysical prospecting from bore-holes extending at anangle from the normal to the earth's surface, as in the case of a nearlyvertical hole in a steeply sloping terrain or a laterally drifted holein a more nearly level terrain, which comprises making a potentialcontact at a subterranean point in the bore hole at the elevation to beinvestigated; making current contacts at the earths surfacesymmetrically located not with respect to the point of penetration ofthe bore hole thereinto, but with respect to the projection of the saidsubterranean point normal to the approximate plane of the earthssurface, so that said current contacts define the base of asubstantially isosceles triangle having the subterranean point at itsapex and of which the bore hole is not a median; passing a currentthrough said current contacts; measuring electrical quantities appearingbetween said subterranean contact and a point at the earths surface inthe current field of, but remote from, the points of current contact;and repeating the said measurement with the surface contacts positionedin the same relative relation to said subterranean contact but insufficient different azimuthal ori- 12 from a pair of current electrodesinserted into a surface region of the earth, when one portion of saiddeeply-buried region is accessible by way of a drill hole or the like,which consists in spacing said current electrodes from each other in oneazitial of the shell crossing the bore hole thereat,

entations relative thereto to provide a contour making another potentialcontact with the earth, at the surface region and spaced away from thedrill hole by a distance approximately equal to one-half the depth ofsaid remotely-buried region, to tap the potential of a shell spaced awayfrom the drill hole and from said current electrodes, and measuring thedifference in potential between said potential contacts; then rotatingsaid configuration of current electrodes and potential contacts to eachof a plurality of other azimuthal directions about said bore-holecontact as a zenithal center and repeating the said measurement in eachof said azimuthal positions of rotation; so that comparing theresistivities indicated by said measurements for the respectiveazimuthal directions of orientation of the configuration can determinethese directions which by maxima and minima resistivities indicate thepresence of geologic extensions and bodies in those portions of theearth spaced away from the bore hole in proximity to said remotelyburiedregion.

. V FREDERICK W. LEE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,951,760 Lee Mar. 20, 19342,153,802 J akosky Apr. 11, 1939 2,160,824 Blau et a1. June 6, 19392,179,593 Jakosky Nov. 14, 1939 2,207,280 Athy et al July 9, 19402,211,124 Jakosky Aug. 13, 1940 2,345,608 Lee Apr. 4, 1944

